Telephone system



Dec. 20, 1938. P v WELCH 2,140,930

TELEPHONE SYSTEM Filed May 1, 1937 WVE/WOR A? M WELCH WWW ATTORNEYPatented Dec. 20., 1938 UNITED STATES PATENT OFFICE TELEPHONE SYSTEMApplication May 1, 1937, Serial No. 140,103

7 Claims.

This invention relates to telephone systems and particularly tosignaling circuits for use in connection with operators cords or otherswitching equipment.

The objects of the invention are to prevent the false operation ofsignaling devices; to render these devices immune to surges and othercurrents present in connecting circuits; to render said signalingdevices sensitive to applied signaling currents notwithstanding thepresence of shunting circuits; to utilize circuits which would otherwiseinterfere with the proper operation of the signaling devices as a meansfor increasing their responsiveness to the signaling currents; andotherwise to improve systems and equipment of this character.

The cord or connecting circuits used in private branchexchanges arefrequently provided with direct current holding bridges connected acrossthe talking conductors of the cord to hold the supervisory equipment atthe central ofiice or at a toli operators position while the privatebranch exchange operator is obtaining a wanted station. It is alsocommon practice to equip these cord circuits with ringing bridges,including such devices as a ringing current responsive relay and acondenser, to enable the central office operator or the toll operator tore-ring the operator in the branch exchange after connection has beenestablished to the branch exchange operators position over one of hercord circuits. The holding bridge, which is closed across the talkingconductors of the cord during certain stages of the operation, acts as ashunt around the ringing bridge, making it diihcult to insure the properoperation of the ringing relay in the presence of the holding bridge.

arises the danger that the ringing relay will respond falsely to thepresence of surges or other currents flowing over the connection and notintended for the operation of the signaling devices.

According to the present invention these difficult s are overcome byproviding the cord circuit nng bridge with a variable resistor, whichautomatically maintainsthe impedance of the ringing bridge high at timeswhen the holding bridge disconnected, and which renders the ringingbridge of low impedance when the holding bridge is connected in shuntrelation therewith. More specifically a resistance element of somematerial having a fairly high negative temperature coefiicient ofresistance is connected in the ringing bridge in series with the ringingrelay therein, and a heating coil for controlling the temperature of fthe impedance of the ringing bridge is chosen to reduce the eifect ofthis shunt, theresaid element is connected in series with the impedancewindings of the holding bridge. The normal high resistance of thevariable-resistance element provides a high impedance for the ringingbridge to safeguard the ringing relay from false operation due to thepresence of intermittent currents in the cord circuit, such as dialingimpulses that may be transmitted by a branch exchange station over thecord circuit. When, however, the branch operator has set up a call fromthe central office or from the toll ofiice to a station line by means ofone of her cord circuits, the holding bridge is connected across theconductors of the cord to provide a path for the flow of direct currentfrom the central office battery to hold the connection until the calledstation answers and to hold the connection after the called partyreplaces his receiver and until the branch exchange operator takes downthe connection to the central oifice. Should the central office operatorattempt to re-ring the branch exchange operator While the holding bridgeis connected in parallel with the ringing bridge, the shunting efiect ofthe holding bridge might prevent the operation or" the ringing relayunless some provision were made to substantially lower the impedance ofthe ringing bridge or improve the sensitivity of the ringing bridge atthis time. Such a reduction of the ringing bridge impedance is obtainedautomatically by providing the variable-resistance element with aheating coil which is connected in series with the holding bridge. Withthis arrangement the direct cur-- rent flowing through the holdingbridge, when connected across the conductors of the cord, heats theresistance element and lowers its resistance to correspondingly lowerthe impedance of the ringing bridge. This arrangement also permits theuse of locking signals without danger of their being locked in falselyby surges on the line.

These and other features of the invention will be described in detail inthe following specification.

. The drawing accompanying this specification illustrates a manualoperators position and connecting cord at a central office or at a tolloffice, a trunk leading therefrom to a branch exchange ofiice, and oneof the cord circuits at the branch exchange operators position forestablishing connections with said lines. One of the branch exchangelines is also illustrated.

Referring to the drawing, the calling subscribers line A is illustratedterminating in a jack at the operators position in the central ofiice.The

operators position in the central ofiice is equipped with the usual cordcircuits, such as cord C, having supervisory relays, answering andringing devices and the other equipment generally associated with suchcords. The operator in the central office has access by way of a trunk Tto a distant private branch exchange. The private branch exchange, whichmay be of the dial type, is provided with operators or attendantspositions for establishing connections either incoming to or outgoingfrom the station lines. One of the operators cord circuits Cl isillustrated, but the switching equipment used for establishingconnections automatically has been omitted for the sake of simplicity.The cord circuit C-l is provided with a ringing or signaling bridge,including the alternating current responsive relay l, condenser 2, and avariable-resistance element 3. The cord C! is also equipped with aholding bridge, comprising a choke coil having a magnetic core- -"l andinductive windings and 6. Connected between the two windings 5 and 6 andin series therewith is a heating coil 1 which is located n proximity tothe resistance element 3 and s to control the temperature thereof.Furhermore the holding bridge includes a key 8 by the operator is ableto disconnect the bridge 11 she wishes to do so for any reason. Forinstance the operator may set up a connection between a station line Band a central ofiice trunk and permit the party at the station line toset up his own connection by manipulating his dial 24 to transmitimpulses through the cord circuit and over the central office trunk tothe switching mechanism in a dial central OffiCE. The holding b" alsopasses through a contact of a supervisory relay 9, and this contactserves to open the when the party at a called station answers, sed loopof the called station replacing the brid e and maintaining thecontinuity of the circuit for supplying current from the central oflice.

While any suitable material may be used for the resistance element 3,there are certain substances, such as boron and silver sulphide, whichare known to have large negative temperature coefficients of resistanceand which are particularly well suited for controlling the resistance ofthe ringing bridge in accordance with the present invention. Aresistance element of this type is disclosed and described in BritishPatent 472,144, accepted Sept. 17, 1937.

The invention has been illustrated in connection with operators cordcircuits used in telephone systems, but it will be understood that theinvention may be applied to systems of other kinds, such as those inwhich connections are established by automatic equipment.

A description of the operation will now be given. Assume that a call hasbeen made by a subscriber at station A ior a connection with the partyat the station line B in the private branch exchange that the centraloilice operator has responded by inserting the answering plug ID of hercord circuit C in the jack H of the calling line. The operator thenmakes connection with the trunk T by inserting plug i2 in the jack I3and causes a calling signal to be displayed at the other end of trunk Tbefore the branch exchange operator. The branch exchange operatorreplies by inserting the answering plug M of her cord C-| in the jack oithe trunk circuit T. The insertion of plug causes the closure of acurrent supply circuit from battery in the central oflice through thelower ri ht winding oi repeating coil 15, winding of supervisory relay16, contacts of ringing key l1, ring contacts of the plug l2 and jackl3, over the lower conductor of trunk T, through the ring contacts ofjack 23 and plug l 4, contacts of supervisory relay 9, key 8, winding 6,heating coil I, winding 5, through the tip contacts of plug l4 and jack23, thence over the tip conductor of the trunk T through contacts of thejack l3 and plug l2, and ringing key N, through the upper right windingof repeating coil l5 to ground. The insertion of plug M in the jack 23serves in the usual manner to efface the calling signal associated withsaid jack. The branch operator connects her telephone set, not shown,across the connection and answers the calling subscriber. Afterobtaining the wanted information, the branch operator disconnects hertelephone set and proceeds to establish the connection by inserting plug[8 of cord C--I into the jack IQ of the wanted station line B. She thentransmits ringing current over the line B to signal the party at thecalled station.

During the time the branch operator is obtaining the called party thebridge circuit above traced is connected across the conductors of thecord Cl, and direct current flows from the central office over thecircuit traced to maintain energized the supervisory relay I6 in thecentral office cord circuit. The flow of current through the bridge andthrough the heating coil 1 raises the temperature of the resistanceelement 3, and this element correspondingly lowers its resistance.Should, therefore, the central office operator desire to recall thebranch exchange operator at any time during this interval, she may do soby operating her ringing key I! to apply ringing current from the source22. Although the holding bridge is closed and is in parallel with theringing bridge at this time the flow of current through the holdingbridge, as described, has reduced the resistance of the element 3 to thepoint where the holding bridge does not shunt enough of the current awayfrom the ringing bridge to endanger the safe operation of the ringingresponsive relay l. The ringing current, therefore, flows through theringing bridge in suflicient quantity to operate the relay l and lightthe lamp 20 as a re-ring signal to the branch exchange operator. Thepresence of ringing current flowing in the unit 3 also acts to raise thetemperature of the element and maintain its resistance at a low value.may be arranged to lock in and remain lighted until the attendantresponds by operating her listening key.

When the party at the called station B answers by removing his receiverfrom the switchhook, a direct current loop is closed in parallel withthe holding bridge, and the current now divides between the holdingbridge and the loop at the called station. The current flowing in theloop of the called station is sufiicient to operate the supervisoryrelay 9, and this relay in operating extinguishes the supervisory lamp,not shown, and also opens the holding bridge to remove the latter fromthe connection. The opening of the holding bridge and the cessation ofcurrent fiow in the coil 1 results in the immediate return of theelement 3 to its normal, high resistance.

At the end of conversation the party at station B replaces his receiver,opening the loop of the called station and permitting relay 9 to releaseand restore the continuity of the holding bridge. Current now continuesto flow from the central office battery through the holding bridge andthe coil 1, and the supervisory equipment at the central office is held,and the resistance unit 3 is This signal iii) heated to lower itsresistance. As before, the central ofiice operator may now recall thebranch exchange operator by transmitting ringing current through thelow-impedance ringing bridge in the cord circuit Cl. When the branchexchange operator notes the disconnect signal, she takes down theconnection by removing the plug M from the jack 23. This releases relayiii in the cord circuit C and gives the usual disconnect signal to thecentral oiiice operator.

In some cases the circuits may be arranged to give the central oflice adisconnect signal when the party at station B hangs up his receiver. Insuch a case any surges caused by the disconnect at the central ofiicewould not operate relay i because of the high resistance of element 3.

What is claimed is:

1. In combination, a line, a cord circuit having two conductors formaking connections with said line, a source of current connected to saidcord, a signaling bridge and a control bridge connected in parallelrelation across the two conductors of said cord, a variable-resistanceelement in said signaling bridge, and means responsive to the flow ofcurrent in said control bridge for varying the resistance of saidelement.

2. The combination in a communication system of a connecting circuit, abridge connected across the conductors of said connecting circuit, asignaling circuit connected in parallel to said bridge, said signalingcircuit having a normally high impedance, a source for supplying currentto said connecting circuit, and means responsive to the flow of currentin said bridge for lowering the impedance of said signaling circuit.

3. The combination in a communication system of a cord circuit, abridgeconnected across the conductors of said cord for holding connections, asignaling bridge connected across said cord and in parallel to saidholding bridge, a variable-resistance element included in said signalingbridge and having a normally high resistance, a source for supplyingcurrent to said cord, a heating coil for said element connected in saidholding bridge and energized by current flowing therein to lower theresistance of said element, and a signal device in said signalingbridge.

4. In combination, a cord circuit having two parallel bridges connectedacross the conductors thereof, a variable-resistance element included inone of said bridges, the resistance of said element varying with itstemperature, means for supplying current to said cord, and means in theother one of said bridges responsive to the flow of current therein forcontrolling the temperature of said element.

5. In combination, a cord circuit having two parallel bridges connectedacross the conductors thereof, a variable-resistance element included inone of said bridges for varying the impedance thereof, a source forsupplying current to the second one of said bridges, means in saidsecond bridge energized by the current therein for varying theresistance of said element, a second source for supplying current tosaid first-mentioned bridge, and signaling means responsive to currentflowing in said first bridge.

6. In combination, a cord circuit, a holding bridge connected across theconductors of said cord, a source for supplying direct current to saidholding bridge, a signaling bridge connected across the conductors ofsaid cord in parallel with said holding bridge, a source for supplyingalternating current to said signaling bridge, means in said signalingbridge responsive to the flow of alternating current therein fordisplaying a signal, a variable-resistance element having a normallyhigh resistance and connected in said ringing bridge, and a heating coilfor said element to raise the temperature thereof and lower itsresistance to alter the sensitivity of said ringing bridge, said heatingcoil being included in said holding bridge and energized by the flow ofcurrent therein.

'7. In combination, a cord circuit having two parallel branchesconnected across the conductors thereof, a variable-resistance element,having a negative temperature coefficient of resistance, included in oneof said bridges, means for supplying current to said cord, and a heatingwinding in the other of said bridges responsive to the flow of currenttherein to control the temperature of said variable-resistance element.

PAUL V. WELCH.

